Raman scattering from ion-implanted diamond, graphite, and polymers.

Abstract

Raman scattering studies were carried out to investigate the effects of ion implantation on the structure of diamond, graphite, and polymers. Carbon phases produced by chemical vapor deposition (CVD diamond) and rf discharge (diamondlike carbon or DLC) were also analyzed. Two types of amorphous carbon phases were distinguished with relevance to hardness. In general, amorphous carbon phases produced by electron beam evaporation and sputtering are soft (hardness \ensuremath{\ll}1 GPa), while DLC and some ion-beam-modified polymers are much harder. In all cases, the characteristic Raman bands of the starting material were lost upon ion implantation, and for the lowest fluences the one-phonon bands near 1360 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ (D line) and 1580 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ (G line) of disordered polycrystalline graphite appeared. With increasing fluence these bands coalesced into a broad, asymmetric peak with the D line shifting to higher wave number and the G line shifting to lower wave number. This trend was clearly distinguishable from the finite crystallite size effect seen in graphite, where, in addition to the appearance of the D line, the G line shifts to higher wave number with decreasing crystallite sizes. Raman scattering could not distinguish between soft and hard amorphous carbon. There was also no indication that the hardness of DLC films and ion-beam-modified polymers was due to diamondlike ${\mathit{sp}}^{3}$ bonds. Instead, hardness in these materials is related to the three-dimensional interconnectivity of chemical bonds. Experimental results suggest that the amorphous carbons examined in this study are composed of random networks of distorted sp, ${\mathit{sp}}^{2}$, and ${\mathit{sp}}^{3}$ bonded atoms, sometimes in a hydrogenated state. The hard carbons such as DLC films and ion beam modified polymers have long-range chemical connectivity while the soft carbons such as damaged graphite, and carbon films prepared by sputter deposition lack such connectivity.